Brazil's government has announced estimates suggesting that deforestation of the Amazon rainforest has fallen by 50% this year.

The government says it believes this is the result of new protection policies.

But environmental groups warn it is too soon to be sure there has been a long-term reversal in the destruction of the world's largest rainforest.

Environment Minister Marina da Silva said some 9,000 sq km (3,475 sq miles) of forest was felled in the last year.

This compares with more than 18,000 sq km (6,950 sq miles) in 2003 to 2004.

Ms Silva said she believed this fall was the result of not only greater government control but also because of more emphasis on sustainable development projects.

Illegal logging crackdown

However, environmental groups, while welcoming the fall, are still treating the announcement with caution.

The figures, they say, are still estimates from satellite images which, because of cloud cover, have a 20% margin of error.

They say a fall in soy prices may also have had an impact, with farmers no longer clearing land.

Finally, they point out that most of the fall in deforestation occurred over a two-month period in June and July this year, when the army and police mounted unusually large operations against illegal logging.

Greenpeace said it was too soon to talk about a long-term slowing of the destruction of the forest, warning that illegal loggers may just be biding their time.

The only firm conclusion, the group said, was that when the government decides to mount major operations against illegal loggers, this does have a positive short-term effect.